In the past, a motivation for deploying multimode fibre was its cheaper cost by comparison with single mode fibre. Unfortunately, multimode fibre has disadvantages over single mode fibre. In particular, signals traversing a multimode fibre will be dispersed or separated into a number of modes. Unfortunately, depending on the length of the multimode fibre, each mode may arrive at the destination at slightly different times. A receiver must compensate for this. Furthermore, the receiver must compensate for ISI (intersymbol interference) found in the received signal.
Recovering data received over a multimode fibre is accomplished using appropriate timing recovery. Many existing timing recovery circuits use a PLL (Phase Lock loop) to determine a proper sampling point. However, as data rates increase, existing timing recovery circuits are no longer adequate for properly recovering data received over multimode fibre. Adaptive equalization plus appropriate timing recovery circuits are used. An entire study group during the 802.3 aq standard was involved in characterizing this problem.
A possible approach is to replace existing multimode fibre with single-mode fibre. However, this approach is generally considered to be impractical because it can be very expensive. There is a very large installed base of multimode fibre. Accordingly, multimode fibre that has been deployed is likely to remain. Therefore, there exists a need for an improved circuit for recovering data received over a multimode fibre that can handle higher data rates.